1. Field of the Invention
The present invention generally relates to a method for mounting an electronic part and a paste material, and more particularly to the method for mounting an electronic part in which a projection electrode of an electronic part is welded by fusion to a connection terminal of a mounting substrate in a flip-chip bonding scheme, and the paste material used in the method.
Recently, as represented by a cellular phone or the like, an electronic part such as a semiconductor device has been improved in terms of reduced size, higher density, and speed. In response to these improvements, a flip-chip bonding scheme has been more frequently used in that a projection electrode such as a solder bump is used as an external connection terminal of the electronic part and the projection electrode is joined to a mounting substrate so as to electrically connect the mounting substrate to the electronic part.
According to this flip-chip bonding scheme, compared with a bonding scheme using lead, since the surface area needed to mount the electronic part can be smaller, it is possible to realize a minimization of the size and a higher density placement. Moreover, it is possible to improve a rate in which a wiring length between the electronic part and the mounting substrate can be shortened.
2. Description of the Related Art
As a flip-chip bonding method for bonding an electronic part having a projection electrode such as a solder bump, a conventional bonding method has been used in that the electronic part is attached to a bonding tool, adjusted at a bonding position on a mounting substrate, and pressed and heated so that a solder bump is contacted and melted so as to be joined. However, since the conventional bonding method individually processes the electric part, the conventional bonding method is less effectively than a surface bonding reflow soldering method.
In contrast to the conventional bonding method, in order to improve operational efficiency, another conventional method similar to a general surface bonding reflow soldering method is also applied in that a solder paste is printed on a connection terminal of the mounting substrate, the solder bump of the electronic part is aligned to be located where the solder paste is printed, and the solder bump is melted by a reflow process to be joined. This method is hereinafter called a first conventional technology. FIGS. 1A, 1B, 1C, and 1D are diagrams showing a mounting method according to the first conventional technology.
In FIG. 1A, a state where a solder paste 5 is printed to a mounting substrate 3 is shown. The solder paste 5 having a volume ratio of a solder grain of about 50% is generally used. This solder paste 5 is arranged on a connection terminal 4 of the mounting substrate 3 using a screen printing method.
Subsequently, a solder bump 2 of a semiconductor device 1 is aligned to the connection terminal 4 of the mounting substrate 3, and as shown in FIG. 1B, the semiconductor device 1 is mounted on the mounting substrate 3. Thereby, the semiconductor device 1 is temporarily fixed on the mounting substrate 3 by the solder paste 5.
Subsequently, the mounting substrate 3 where the semiconductor device 1 is temporarily fixed is passed through a reflow furnace, and the solder bump 2 is melted to join to the connection terminal 4. In FIG. 1C, the solder bump 2 is joined to the connection terminal 4. When the solder bump 2 is completely joined to the connection terminal 4 by the reflow process, unremoved flux is washed off if needed. As shown in FIG. 1D, the semiconductor device 1 is completely mounted to the mounting substrate 3.
On the other hand, by utilizing a fact that the bump itself is solder, another method is also applied in that only flux is coated on a mounting substrate or at an end of a solder bump, and the electronic part is mounted on a mounting substrate by the reflow process. This method is hereinafter called a second conventional technology. Another mounting method will be described according to this second conventional technology with reference to FIGS. 2A, 2B, 2C and 2D. Also in FIGS. 2A, 2B, 2C, and 2D, an example using the semiconductor device 1 as the electronic part is shown.
FIG. 2A is a diagram showing a state where flux 18 is printed to a mounting substrate 13. Different from the solder paste 15 in FIG. 1A, the flux 18 does not contain the solder grain. The flux 18 is arranged to completely cover the upper surface of the mounting substrate 13.
Subsequently, the solder bumps 12 of the semiconductor device 11 are adjusted to be aligned to the connection terminals 14 of the mounting substrate 13, respectively. As shown in FIG. 2B, the semiconductor device 11 is mounted on the mounting substrate 13. Thereby, the semiconductor device 11 is temporarily joined to the mounting substrate 13 by the flux 18.
The mounting substrate 13 where the semiconductor device 11 is mounted on the mounting substrate 13 is passed through the reflow furnace. The solder bumps 12 are melted to join to the connection terminals 14. FIG. 2C is a diagram showing a state where the solder bumps 12 are joined to the connection terminals 14. When the solder bumps 12 are completely joined to the connection terminals 14 by the reflow process, residue flux is washed off if needed. Thereby, as shown in FIG. 2D, the semiconductor device 11 is completely mounted to the mounting substrate 13.
However, in the first conventional technology, a technology is required to minutely print each solder paste 5 on each connection terminal 4 of the mounting substrate 3. In a case in which the printing process is not properly performed, a bridge part 6 between the connection terminals 4 is produced (see FIG. 1C and FIG. 1D). Also, a poor joint between the solder bump 2 and the connection terminal 4 is caused.
In the solder paste 5 whose volume ratio of the solder grains is about 50%, in a case in which a bump pitch of the semiconductor device 1 is less than 150 micrometers, it is difficult to properly print each solder paste 5 to each minutely formed connection terminal 4 corresponding to this bump pitch. Furthermore, the semiconductor device 1 being mounted must be stably attached at a predetermined mounting position on the mounting substrate 3 until the reflow process is completed. However, it is difficult for a minute amount of the solder paste 5 printed on connection terminals 4 to sufficiently maintain the attachment.
On the other hand, in the second conventional technology described above, since an allowance for a flatness of the solder bump 2 in relation to the mounting substrate 3 is small, the connection terminals 4 of the mounting substrate 3 may not be joined with the solder bumps 2. That is, a variation in a diameter inevitably exists in the solder bumps 2 (shown by an arrow xcex94H in FIG. 2A). For this reason, such as a solder bump 12B shown in FIG. 2A, if a diameter of the solder bump 12 is smaller than that of a normal solder bump 12A, a space occurs between the solder bump 2B and the connection terminal 4.
In the mounting method according to the first conventional technology shown in FIG. 1A through FIG. 1D, since the solder paste 5 contains solder grains as about 50% of the volume ratio of the solder paste 5, even if the space occurs between the solder bump 2 and the connection terminal 4 when the solder grain fuses in a heating process, fused solder grain fills the space. Accordingly, the space as a problem does not occur in the mounting method according to the first conventional technology.
However, in the mounting method according to the second conventional technology shown in FIG. 2A through FIG. 2D, since the solder grain does not exist in the flux 18, when a space is formed between the solder bump 12B and the connection terminal 14 because of the variation of a diameter of the solder bump 12B, a state where an interval is defined between the solder bump 12B and the connection terminal 14 is maintained even in the reflow process. As a result, the interval causes a poor connection between the solder bump 12B and the connection terminal 14.
In order to solve problems described above, Japanese Laid-Open Patent Application No. 4-262890 discloses that a semiconductor device (solder bumps are not provided) and a mounting substrate are joined by a thermosetting adhesive bond containing a solder grain and flux. Moreover, Japanese Laid-Open Patent Application No. 11-186334 discloses that an anisotropic conductive material, in which the solder grain having the flux is contained in a thermosetting resin sheet or paste, is used and a mounting substrate is joined to a semiconductor device by this anisotropic conductive material.
However, in these disclosed mounting methods, it is required to select a thermosetting adhesive bond, a thermosetting resin sheet, or an anisotropic conductive material (hereafter generally called thermosetting resin) in order to tolerate the heating process in a soldering process. Furthermore, since this thermosetting resin has a function as a so-called under fill resin, this thermosetting resin is influenced by a combination of a size of a semiconductor device, a space of the semiconductor device and the mounting substrate, an electrode material, a mounting substrate material, and the like. Accordingly, in order to secure mounting reliability, it is required to adjust material characteristics such as an elasticity modulus, a thermal expansion coefficient, and such as an adhesive strength. However, to select the thermosetting resin, it is required to consider both the heat resistance in the heating process and the characteristic as the under fill resin. Therefore, it is complicated to select a suitable thermosetting resin.
It is a general object of the present invention to provide a method for mounting an electronic part and a paste material in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a method for mounting an electronic part and a paste material, in which a mounting reliability between projection electrodes and connection terminals can be improved when the electronic part is mounted on the mounting substrate.
The above objects of the present invention are achieved by a method for mounting an electronic part on a mounting substrate in that projection electrodes provided on the electronic part are welded by fusion to join connection terminals provided on the mounting substrate, the mounting method including the steps of: arranging a flux paste on the mounting substrate, the flux paste including a base flux and metal grains having diameters smaller than diameters of the projection electrodes and having a thickness so as to form a space between the flux paste and the electronic part when the electronic part is mounted on the mounting substrate; and sealing a resin in the space formed between the electronic part and the mounting substrate after the projection electrodes are joined to the connection terminals.
According to the present invention, even if a space occurs between the top of the projection electrode and the connection terminal in joined positions by variation in the diameter (height) due to the projection electrode, since the flux paste including the metal grains and the base flux is arranged on the mounting substrate, the metal grains existing in the space assist in joining the projection electrode to the connection terminal. That is, even if the space exists between the projection electrode and the connection terminal, the metal grains connect the space electrically. Therefore, it is possible to prevent a poor joining from occurring between the projection electrode and the connection terminal. In this case, since the diameters of the metal grains are defined as smaller than the diameters of the projection electrodes, a distance from the electronic device to the mounting substrate cannot be more than the diameters of the projection electrodes due to the metal grains.
Moreover, since the space formed between the electronic part and the mounting substrate is filled up with the resin, it is possible to improve a mounting strength between the electronic part and the mounting substrate. It is possible to easily select the resin arranged at this time without considering the quality of the material of the flux paste.
In the method described above, the diameters of the metal grains may be defined to be more than a tolerance of diameters of the projection electrodes and less than one third the diameters of the projection electrodes, and the flux paste may include the metal grains more than one percent and less than twenty percent by volume ratio.
According to the present invention, since the diameters of the metal grains are defined smaller than the tolerance of the diameters of the projection electrodes, even if the space occurs between the projection electrodes and the connection terminals due to the variation in the diameters of projection electrodes, the connection terminals can be electrically connected to the projection electrodes securely.
That is, when the electronic part is mounted on the mounting substrate, the space, which occurs between the projection electrodes and the connection terminals due to the variation in the diameters of the projection electrodes, becomes at maximum in the tolerance of the diameters of the projection electrodes. Thus, by defining the diameters of the metal grains as more than the tolerance of the diameters of the projection electrodes, namely, by defining the diameters of the metal grains more than a maximum space that may occur, the projection electrodes can be securely electrically connected to the connection terminals.
Moreover, by defining the diameters of the metal grains as less than one third the diameters of the projection electrodes, when fusing the projection electrodes and joining the projection electrodes to the connection terminals, it is possible to prevent adjacent projection electrodes from short-circuiting by connecting to the metal grains.
Furthermore, the flux paste includes the metal grains more than one percent and less than twenty percent by the volume ratio. Compared with a conventional solder paste which is generally used and includes the solder grains about fifty percent by the volume ratio, it is possible to suppress the occurrences of bridge defects without severely controlling an arrangement amount. In addition, it is possible to securely maintain the electronic part mounted on the mounting substrate until the projection electrodes are joined by fusion to the connection terminals.
In the method described above, the flux paste may be arranged on an area having the connection terminals of the mounting substrate by a thickness being more than the diameters of the metal grains and less than half the diameters of the projection electrodes.
According to the present invention, the flux paste is arranged on the mounting substrate by a thickness that is more than the diameters of the metal grains and less than half the thickness of the projection electrodes. Thus, when the joining process is completed, it is possible to prevent the space between the electronic part and the mounting substrate from being filled up with the unremoved flux. Therefore, it is possible to easily conduct the sealing process by the resin after the joining process.
In the method described above, a base flux of the flux paste may include a resin used to seal the space as a main constituent.
According to the present invention, the base flux that remains after the joining process has the same constituents as the resin arranged between the electronic part and the mounting substrate after that, a connectivity to the resin for sealing is favorable. Therefore, it is not required to wash the base flux off before arranging the resin.
Alternatively, in the method described above, the base flux of the flux paste may include a constituent in which filler is eliminated from a resin used to seal the space, as a main constituent.
According to the present invention, when the projection electrodes are joined to the connection terminals, since the filler of the resin described above does not exist between the projection electrodes and the connection terminals, it is possible to improve the connectivity.
Moreover, in the method described above, the projection electrodes may have a spherical shape, and the metal grains may have a shape having a smooth surface so as to be movable when the projection electrodes are pressed to the connection terminals.
According to the present invention, the electronic part is mounted on the mounting substrate, the metal grains move smoothly between the projection electrodes and the connection terminals while the projection electrodes approach the connection terminals. Even if the space exists between the projection electrodes and the connection terminals, the metal grains electrically connect the projection electrodes and the connection terminals securely in the space. Therefore, it is possible to improve the mounting reliability of the projection electrodes and the connection terminals.
The above objects of the present invention are achieved by a method for mounting an electronic part on a mounting substrate in that projection electrodes provided on the electronic part are welded by fusion to join connection terminals provided on the mounting substrate, the mounting method including the steps of: arranging a flux paste on the projection electrodes, the flux paste including metal grains having diameters smaller than diameters of the projection electrodes and having a thickness so as to form a space between the electronic part and the flux paste when the electronic part is mounted on the mounting substrate; joining the projection electrodes to the connection terminals by mounting the electronic part on the mounting substrate and conducting a heating process; and sealing a resin in the space formed between the electronic part and the mounting substrate after the projection electrodes are joined to the connection terminals.
According to the present invention, since the metal grains electrically connect the projection electrodes to the connection terminals in the space even if the space exists between the projection electrodes and the connection terminals, it is possible to prevent occurrences of joint defects between the projection electrodes and the connection terminal. In addition, since the resin for sealing fills up between the electronic part and the mounting substrate, it is possible to improve the mounting strength between the electronic part and the mounting substrate.
In the present invention, the diameters of the metal grains may be defined to be more than a tolerance of the diameters of the projection electrodes and less than one third the diameters of the projection electrodes, and the flux paste includes the metal grains more than one percent and less than twenty percent by volume ratio.
According to the present invention, even if the space occurs between the projection electrodes and the connection terminals due to the variation in the diameters of the projection electrodes, it is possible to electrically connect the projection electrodes to the connection terminals. In addition, the diameters of the metal grains are defined less than one third the diameters of the projection electrodes. Therefore, it is possible to prevent adjacent projection electrodes from short-circuiting by the metal grains.
Furthermore, the flux paste includes the metal grains more than one percent and less than twenty percent by the volume ratio. Thus, it is possible to suppress the occurrences of a bridge defects without severely controlling the arrangement amount. Also, it is possible to securely maintain the electronic part mounted on the mounting substrate until the projection electrodes are joined by fusion to the connection terminals.
The above objects of the present invention are achieved by a paste material used to weld by fusion projection electrodes provided on an electronic part to connection terminals provided on a mounting substrate, the paste material including: metal grains; and a base flux, wherein diameters of the metal grains are defined to be more than a tolerance of diameters of the projection electrodes and less than one third the diameter of the projection electrodes, and the metal grains are included more than one percent and less than twenty percent by volume ratio.
According to the present invention, even if the space occurs between the projection electrodes and the connection terminals due to the variation in the diameters of the projection electrodes, the connection terminals are electrically connected to the projection electrodes securely. In addition, the diameters of the metal grains are defined less than one third the diameters of the projection electrodes. Thus, when the projection electrodes are joined by fusion to the connection terminals, it is possible to prevent adjacent projection electrodes from short-circuiting by the metal grains.
Moreover, the flux paste includes the metal grains more than one percent and less than twenty percent by the volume ratio. Thus, it is possible to suppress the occurrences of the bridge defects without severely controlling the arrangement amount. Also, it is possible to securely maintain the electronic part mounted on the mounting substrate until the projection electrodes are joined by fusion to the connection terminals.